The present invention will be described in the context of an implantable cardiac control device such as a defibrillator or cardioverter for processing tachycardia and tachyarrythmia events, but it can also be applied to cardiac stimulation devices (also known as pacemakers) for processing bradycardia events, which two kinds of devices are often combined in a single implanted machine (often referred to as an antitachycardia pacemaker). More particularly, the present invention concerns circuits and methods for detecting the occurrence of a cardiac depolarization in the tissue adjacent to one or more electrodes in contact with or in proximity to the heart, to provide for the determination and analysis of the cardiac rhythm.
Circuits for detecting cardiac signals, which have an automatic adjustment of the sensitivity threshold for detecting the signals, are known. As used herein, the terms "sensing threshold", "detection threshold", and "sensitivity threshold" are used interchangeably to indicate the threshold amplitude above which a signal coupled from the heart, under the conditions described herein, is determined to be a "sensed signal". The terms "sensed signal" and "detection" are used interchangeably to refer to a signal corresponding to a cardiac depolarization complex.
A problem with these known circuits is that the sensitivity threshold is automatically set either too low to avoid detection of noise or other spurious interference signals, or too high to detect certain cardiac depolarizations, and not at a level that adequately rejects noise and ensures detection of cardiac depolarizations. Several of the known prior art techniques addressing this problem are as follows.
Langer U.S. Pat. No. 4,184,493 (filed February 1978) refers to a sensing circuit for an implantable defibrillator, having an amplifier, a first-order high-pass filter (the corner frequency of which is undisclosed), a bidirectional comparator, and a circuit which measures the fraction of time that the comparator asserts its output. The detector ultimately measures the average fraction of time that the ECG signal has a high slope compared to the time it has a low slope The circuit provides feedback to adjust amplifier gain automatically to maintain constant amplitude at filter output.
Imran U.S. Pat. No. 4,393,877 (filed May 1981) refers to a sensing circuit for an implantable defibrillator, which includes a slew rate detector circuit and a zero-crossing detector circuit. The slew rate detector has an amplifier, a first-order high-pass filter (corner frequency undisclosed), a unidirectional comparator, and a monostable multivibrator for timing a refractory period. The Imran circuit provides feedback to adjust amplifier gain automatically to maintain constant amplitude at filter input.
Mower U.S. Pat. No. 4,559,946 (filed June 1982) also refers to a sensing circuit synchronizing shocks to cardiac activity in an implantable defibrillator, which includes a slew rate sensitive circuit followed by a comparator. The Mower circuit provides feedback to adjust the comparator threshold automatically to a fraction of recent peak filter output.
Menken U.S. Pat. No. 4,819,643 (filed November 1986) discloses a defibrillator with pacing and sensing functions, and a separate fibrillation sensing channel with automatic gain control, which retards pacing (stimulation) until the fibrillation sensing channel gain reaches maximum sensitivity.
Grevis U.S. Pat. No.4,940,054 (filed April 1988) refers to a defibrillator with at least two values of sensitivity, which are switched depending on the type of detected rhythm.
Baker U.S. Pat. No. 4,880,004 (filed June 1988)refers to a sensing circuit for arrhythmia detection, which includes an input amplifier with an unspecified bandpass, followed by a sensing channel with a high-pass filter at 25 Hz, and a measuring channel with additional high-pass filtering at 12 Hz, each channel followed by comparators. The Baker circuit provides feedback to adjust amplifier gain automatically to maintain constant sensing margin (the amount by which the signal in the sensing channel exceeds its threshold). Due to the additional filtering in the measuring channel this margin improves at low frequencies.
Carroll U.S. Pat. No. 4967747 (filed May 89) refers to an implantable defibrillator with a sensing circuit, consisting of a switched capacitor gain/filter block with unspecified bandpass, designed to provide glitch-free (smooth) gain changes for automatic gain control.
Keimel U.S. Pat. No. 5,117,824 (filed November 1990) refers to an implantable pacing and sensing circuit with automatic threshold control, which does not adjust the threshold after pacing, and which sets threshold according to measured amplitude after sensing, gradually reducing threshold thereafter.
The question of adjusting the sensitivity, particularly for defibrillators and cardioverters (but not exclusively) has not resulted in a satisfactory solution and remains the subject to a number of controversies.
In "State-of-the-Art of the AICD", Pace May 1991 Winkle described a defibrillator without automatic gain control, for which "programming sensitivity requires great care at time of implantation and during follow-up to be certain that T waves are not oversensed and most importantly that ventricular fibrillation/tachycardia is not undersensed".
In "Failure of a Second and Third Generation Implantable Cardioverter to Sense Ventricular Tachycardia: Implications for Fixed-Gain Sensing Devices", Pace, May 1992, Sperry et al. wrote that "more sensitive fixed-gain settings or automatic-gain sensing are needed to detect low amplitude signals on a consistent basis. The undesirable aspect of using such high sensitivity is that of oversensing (e.g., T waves) by the device resulting in an increased risk of inappropriate discharges. Failure to sense ventricular fibrillation has however also been reported with automatic gain devices as well".